1. Field of the Invention
The present invention relates to image processing systems. More specifically, the present invention relates to systems and methods for image registration.
2. Description of the Related Art
In applications such as remote sensing, medical imaging, and computer vision, multiple images of a scene or object are acquired at different times or from different perspectives. These images will typically be in different coordinate systems. Image registration is the process of transforming the images such that they are all in one coordinate system, allowing for the comparison or integration of data from the different images. The transformation used may include translation, rotation, scaling, or other more complicated transforms.
In general, image registration can be performed by trying different transforms on the target image (for example, translating the image using several different shifts), computing a correlation metric for each transform, and selecting and applying the transform having the highest correlation. This process, however, -an become computationally expensive as the size of the image increases. The problem is even greater when sub-pixel registration is required (i.e., using image shifts of a fraction of a pixel).
A commonly used solution is the Fitts' algorithm, which uses a gradient-based approach to perform sub-pixel image registration. Gradient-based approaches are typically less computationally expensive than correlation approaches, but can be less accurate. The Fitts' algorithm is typically only accurate to within one pixel. As is known in the art, its performance is typically unsatisfactory, for images having shifts of more than one pixel. Applying low pass filtering followed by downsampling prior to applying the Fitts' algorithm does not always yield satisfactory results. Low pass filtering and downsampling increases the dynamic range at the expense of precision. This low pass filtering and downsampling approach is limited by the amount of detail in the image. It is possible to remove valuable information with this approach and, from experimentation, it does not necessarily result in the correct image shift.
Hence, a need exists in the art for an improved system or method for sub-pixel image registration offering greater accuracy and shorter computation time than prior approaches.